This invention relates to the correction of rotor unbalance, and is particularly, although not exclusively, concerned with a correction of unbalance of a rotor in the form of a bladed disc, or “blisk” of a gas turbine engine.
Components that rotate at high speed must be balanced if undesirable vibration and associated stresses are to be avoided. This is particularly the case for rotors of gas turbine engines.
In the past, bladed discs have been assembled by fitting individual blades to a central rotor disc. It has been possible to balance the assembled discs by using the mass variation between individual blades, so that the positions of individual blades can be changed to eliminate any out-of-balance that occurs. More recently, integral bladed discs have become more common. Such bladed discs are referred to as “blisks”. With such structures, it is not possible to exchange individual blades, and so a different balancing method is required.
A rotating out-of-balance can take two forms. The first is static unbalance in which the centre of mass lies away from the rotational axis so that, if the rotor is supported in a frictionless manner, it will always come to rest in a position in which the centre of mass lies directly below the axis. A known method of correcting static unbalance is either to add an appropriate mass to the rotor at a position diametrically opposite the detected centre of mass, or to remove material from the same side of the axis as the centre of mass. The effect of such adjustments is to move the centre of mass to the rotational axis.
The second kind of rotating out-of-balance is dynamic unbalance which arises if out-of-balance forces are generated in different directions at different positions along the rotational axis of the rotor. Such forces give rise to a rotating couple unbalance when the rotor is rotated, and this can cause problems even if the rotor is in static balance.
As with static unbalance, it is known to correct couple unbalance by the addition or removal of material from the rotor. In both cases this involves a weight penalty, either from the direct effect of adding a balancing mass, or because the removal of material means that balancing lands need to be provided from which controlled material removal can be effected, and these balancing lands add to the weight of the component as a whole.
It is also known to correct static unbalance by making corrections during the final machining of location features, but such methods do not correct dynamic unbalance.